1. Field of the Invention
The present invention relates to an optical pulse time spreading device that is suitable for use as an optical encoder or optical decoder that is employed in an optical code division multiplexing transceiver device of a time-spreading and wavelength-hopping system.
2. Description of Related Art
In recent years, the demand for communications has increased rapidly as a result of the popularization of the Internet and so forth. High-speed and high-capacity networks using optical fiber have accordingly been completed. Further, in order to establish high-capacity communications, optical multiplexing technology that transmits a plurality of channels' worth of optical pulse signals together via one optical fiber transmission line has become essential.
As optical multiplexing technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and optical code division multiplexing (OCDM) have been intensively researched. Among these technologies, OCDM has the merit of flexibility on the operation side in having no restrictions on the time axis allocated one bit at a time for optical pulse signals that are transmitted and received in OTDM and WDM and so forth. Further, OCDM has the merit that a plurality of channels can be established in the same time slot on the time axis or a plurality of communication channels can also be established with the same wavelength on the wavelength axis.
OCDM is a communication method that allocates different codes (patterns) to each channel and extracts signals by means of pattern matching (See S. Kutsuzawa, et al., “10 Gb/s×2 ch Signal Unrepeated Transmission Over 100 km of Data Rate Enhanced Time-spread/Wavelength-Hopping OCDM Using 2.5-Gb/s FBG En/Decoder” IEEE Photonics Technology Letters, Vol. 15, No. 2 pp. 317-319, February 2003 or Hideyuki Iwamura, et al. “FBG based Optical Code En/Decoder for long distance transmission without dispersion compensating devices” Optical Society of America 2004 WK6, for example). That is, OCDM is an optical multiplexing technology that encodes an optical pulse signal by means of optical codes that are different for each communication channel on the transmission side and which restores the original optical pulse signal by performing decoding by using the same optical codes on the reception side as on the transmission side.
With OCDM, because only an optical pulse signal that matches the code when the optical pulse signal has been encoded is extracted and processed as an effective signal during decoding, an optical pulse signal that consists of light rendered by combining the same wavelength or a plurality of wavelengths can be allocated to a plurality of communication channels. Further, with OCDM, because the same code as the code used for encoding must be used in order to perform decoding on the reception side, decoding is not performed unless the optical code is known. Hence, OCDM is a transmission method that is also superior in the stability and security of information.
A passive light element that does not consume power such as a Superstructured Fiber Bragg Grating (SSFBG) or an Array Waveguide Grating (AWG) can be used as the optical encoder. Hence, an increase in the communication rate is possible without receiving an electrical processing speed restriction. Further, a plurality of channels can be multiplexed at the same time and same wavelength and large-capacity data communications may be performed. That is, in comparison with OTDM and WDM and so forth, the focus is on the fact that the communication capacity can be rapidly increased.
Time spreading and wavelength hopping systems are known as encoding means. When time spreading and wavelength hopping systems are applied to OCDM, encoding that considers not only time but also wavelength is performed. Time spreading and wavelength hopping systems will appear as ‘time spreading/wavelength hopping method’ hereinbelow. Further, the code used in the time spreading and wavelength hopping systems will appear as ‘time spreading/wavelength hopping code’.
OCDM which uses the time spreading/wavelength hopping method is a transmission method that is performed via the following steps. First, on the transmission side, the output of a multiple wavelength continuous wave light source or wide bandwidth light source is converted into an optical pulse train and, based on this optical pulse train, a transmission signal constituting a binary digital signal is converted into an RZ (return to zero) optical pulse signal to generate the optical pulse signal to be transmitted. This optical pulse signal is transmitted after being encoded by the optical encoder. Meanwhile, on the reception side, the transmitted optical pulse signal is played back as a result of decoding by the optical decoder for which the same code as the code set for the optical encoder above has been set.
In the case of OCDM that uses the time spreading/wavelength hopping method, the optical pulse on the time axis constituting the RZ optical pulse signal is constituted comprising light of a plurality of wavelengths and one optical pulse is wavelength-divided by the optical encoder and arranged spread on the time axis. Further, the same wavelength components are also similarly arranged spread on the time axis by the optical encoder in accordance with fixed regulations (code set for the optical encoder). Hence, the time-spreading/wavelength hopping method has the benefit that encoding is possible by means of two degrees of freedom which are time and wavelength. As a result, in comparison with a case where encoding is performed by means of the time-spreading method with an optical pulse signal consisting of a single wavelength serving as the target of the encoding, encoding in which wavelength is also considered can be executed and there is therefore the merit that confidentiality can accordingly be improved.
As mentioned earlier, in the time-spreading/wavelength hopping method, the element fulfilling the role of arranging the optical pulse constituting the optical pulse signal on the time axis through wavelength-division is the optical encoder. Thereafter, an optical pulse (an optical pulse of a single wavelength) dispersed on the time axis in this manner is also called a chip pulse. The chip pulse dispersed on the time axis is decoded by the optical decoder to obtain the optical pulse (an optical pulse containing a plurality of wavelengths) that constitutes the original optical pulse signal.
Thus, the optical encoder fulfils the role of breaking down the optical pulse constituting the optical pulse signal into chip pulses and spreading same on the time axis and is therefore also known as an optical pulse time spreading device. Further, the optical decoder fulfils the role of restoring the chip pulses to an optical pulse constituting the original optical pulse signal and therefore fulfils a role that is the reverse of that of the optical encoder. However, because the structure of the optical decoder is the same by virtue of this being an element for which the same code has been set, the optical decoder is also likewise called an optical pulse time spreading device. Therefore, when either the optical encoder or optical decoder is indicated in the subsequent description, either can also be represented as an ‘optical pulse time spreading device’.
When utilized in an OCDM system, the roles of the optical encoder and optical decoder are determined by the point in which the optical encoder and optical decoder are disposed in the system. The time-spreading/wavelength hopping code set for both the optical encoder and optical decoder is the same. That is, if disposed on the transmission side, the optical pulse time spreading device functions as an optical encoder, whereas, if disposed on the reception side, the optical pulse time spreading device functions as an optical decoder.
An SSFBG, which is used as an optical pulse time spreading device in OCDM that uses the time-spreading/wavelength hopping method, is constituted by disposing a single Fiber Bragg Grating (FBG) with a Bragg reflection wavelength equal to the wavelength of light of a plurality of wavelengths constituting one optical pulse on the time axis. For example, when one optical pulse is constituted by the wavelengths λ1, λ2, λ3, and λ4, the SSFBG is constituted by arranging single diffraction gratings with the Bragg reflection wavelengths λ1, λ2, λ3, and λ4 respectively. The arrangement pattern of the signal diffraction gratings is decided by the codes set for the optical pulse time spreading device.
Apart from the SSFBG above, an element that is formed by connecting a power splitter, a thin-film filter, and a time delay section (See Varghese Baby, et al. “Experimental Demonstration and Scalability Analysis of a Four-Node 102-Gchip/s Fast Frequency-Hopping Time-Spreading Optical CDMA Network” IEEE Photonics Technology Letters, Vol. 17, No. 1 pp. 253-255, January 2005, for example) can also be used as the optical pulse time spreading device. Although such an element provides the benefit of not imposing restrictions on the code that can be set, on the other hand the light exposure is large in comparison with that of the SSFBG and there is also the problem that miniaturization of the whole element is difficult. Therefore, the focus was on usage of the SSFBG as the optical pulse time spreading device that was used in the optical code division multiplexing transceiver device.
As mentioned earlier, the SSFBG imposes a certain restriction on the time-spreading/wavelength hopping code that can be set. Such a restriction is not imposed on the optical pulse time spreading device formed by connecting a power splitter, thin-film filter, and time delay section. This certain restriction is a restriction that the relationship between the chip rate and chip size of the optical pulse time spreading device must be established so that the pulse width on the time axis with respect to one optical chip pulse is not wider than the minimum time interval between adjacent optical chip pulses. Subsequently, for the sake of expediency in the description, the minimum time interval of adjacent optical pulses will also be called the ‘chip cycle’.
When the chip size is longer than the chip cycle, adjacent optical chip pulses that have been subjected to time-spreading/wavelength hopping encoding by the optical pulse time spreading device produces parts that overlap on the time axis. Hence, a situation where the two chip pulses cannot be completely wavelength-divided arises at the stage where the chip pulses are decoded.
In order to avoid such a situation, it is necessary to first widen the center wavelength interval of the spectral of light of a plurality of different wavelengths contained in one optical pulse forming the optical pulse signal. However, because the wavelength bandwidth of light that can be used in optical communications is in a limited range, this places restrictions on the widening of the center wavelength interval of the spectral.
Furthermore, the wavelength discrimination sensitivity of the optical pulse time spreading device must be raised. That is, the halfwidth of the output light output from the optical pulse time spreading device with respect to the wavelength must be narrowed. However, in order to narrow the half width of the output light with respect to wavelength, the FBG unit constituting the SSFBG used as the optical pulse time spreading device must be lengthened and the overall length of the optical pulse time spreading device is long, which constitutes an obstacle to practical use.